-LRB- CNN -RRB- -- The chilling acts the 15-year-old boy is accused of defy imagination :

Pumping his mother , brother and two younger sisters with bullets .

Gunning down his dad when he returned home .

Texting a picture of his lifeless mother to his 12-year-old girlfriend .

Plotting to kill strangers outside a supermarket .

But , family members say , Nehemiah Griego is no monster . They ca n't fathom what could have gone so terribly wrong .

`` Whether it was a mental breakdown or some deeper undiagnosed psychological issue , we ca n't be sure yet , '' his uncle , former New Mexico state Sen. Eric Griego , said .

`` What we do know is that none of us , even in our wildest nightmare , could have imagined that he could do something like this . ''

The family statement describes the boy as a gifted athlete and musician who was active in youth services at church .

It said the teen accompanied his father on mission trips to Mexico and wanted to continue a long family history of military service by becoming a soldier .

The family disputed any media characterization that the teen was a loner and said he had many friends and was a wrestler in state tournaments . His father was a champion wrestler and coach .

The family also said they did not want anyone to use the case in the nation 's unfolding debate about guns .

`` To be clear , our family has differing views on gun rights and gun control . What we do agree on is that those who wish to score political points should not use a confused , misguided , 15-year-old boy to make their case , '' the family statement said .

`` We ask those in the media and those who would use the media to make their political case , to not use Nehemiah as a pawn for ratings or to score political points . He is a troubled young man who made a terrible decision that will haunt him and his family forever . ''

But authorities painted a far different picture of the teen .

Sheriff : Teen planned shootings for at least a week

Bernalillo County Sheriff Dan Houston said Griego was `` involved heavily '' with violent video games and that he `` was quite excited as he got the opportunity to discuss that with our investigators . ''

He said that the boy planned the shootings for at least a week . That he told investigators he was frustrated with his mother . That he contemplated killing his girlfriend 's parents . That he hoped to continue his killing spree at a Walmart `` with mass destruction '' and die in a shootout with police .

The teen 's demeanor , as he shared all this , was `` stern , very unemotional , '' Houston said .

Now , Griego will stand trial as an adult in the weekend killings , prosecutors announced .

He was arrested Saturday night , after deputies found the bodies of his mother , father , brother and two of his sisters in their home on the outskirts of Albuquerque .

Opinion : How a boy becomes a killer

The tragedy unfolds

The tragedy unfolded early Saturday morning , when Griego carried a .22 - caliber rifle to where his mother and 9-year-old brother slept side by side , an arrest affidavit said .

The teen fatally shot his mother , waking his younger brother Zephania in the process .

When Griego told Zephania that their mother was dead , the younger boy did not believe him , the document stated .

`` So Nehemiah picked up his mother 's head to show his brother her bloody face , '' the affidavit said . `` Nehemiah stated his brother became upset , so he shot his brother in the head with the same rifle used to kill his mother . ''

After that , police said , `` Nehemiah stated he lost his sense of conscience . ''

The boy went on to kill his sisters , 5-year-old Jael and 2-year-old Angelina , before his father Greg came home from a volunteer graveyard shift at a homeless shelter .

Authorities believe the father , a former pastor at an Albuquerque church and a chaplain to the city 's fire department , was the last to be gunned down in the home .

KRQE : Security officer -- Boy was a ` great kid '

` Beyond any human reasoning '

Most of the victims were shot with the .22 , but a semiautomatic AR-15 rifle was also used in the killings , sheriff 's Lt. Sid Covington said . Authorities said the weapons appear to have been purchased legally .

The Griegos also had five grown children , none of whom lived at home , Covington said .

After the slayings , Griego `` spent the majority of the day with his girlfriend , '' Houston said .

He even sent his girlfriend a picture of his slain mother , Houston said .

Authorities arrested the teen after he attended his family 's church with his girlfriend , and a staff member called deputies .

Pastor Skip Heitzig said father Greg Griego was also a pastor for a number of years who did prison ministry at the Metropolitan Detention Center and helped to rehabilitate convicts . Greg Griego also helped them re-enter society .

`` The news stunned us all , '' Heitzig said of the shooting .

`` We are doing what we can as a church body to minister to the remaining family members . Only the Lord Jesus Christ can heal this type of pain and heartache , and He is faithful to do so , '' he added in a statement .

His church , Calvary Albuquerque , is scheduled to hold a prayer vigil Wednesday at 7 p.m. for the Griego family .

Griego was scheduled to appear in court Tuesday to face two counts of murder and three counts of child abuse resulting in death , but the hearing was waived , CNN affiliate KOAT reported .

The affiliate said Griego 's case will go to a grand jury within 10 days .

`` This is beyond any human reasoning or understanding at this time , '' the sheriff said .

CNN 's Leslie Holland and Michael Martinez contributed to this report .

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NEW : Family asks public not to use the incident in national debate on guns

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NEW : `` To be clear , our family has differing views on gun rights and gun control , '' they say

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Griego will face charges as an adult in the deaths of his parents and three siblings

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`` This is beyond any human reasoning or understanding at this time , '' the sheriff says